1. Field of the Invention
The present invention relates to an improved carbon for use in electrodes for a non-aqueous electrolyte secondary batteries and other similar energy storage devices. More particularly, it is concerned with an improvement in the electrode which includes an electrode active material being capable of reversibly intercalating and deintercalating lithium.
2. Description of the Prior Art
The present invention is drawn to intercalating carbon materials, generally. For many years manufacturers of batteries, especially high energy density lithium batteries, have searched for an electrode material which would allow them to produce safe, inexpensive, secondary cells having a high cycle life, high rate capability, and high voltage. Very early designs proposed using a material having a layered crystal structure which would allow lithium ions to migrate into and out of the interstices of the crystal lattice. The first of these materials to be successfully commercialized was a cell based on molybdenum disulfide. However, it soon became apparent that this material posed unacceptable safety problems.
Much later, carbons of various configuration were proposed as an alternative. These materials have been widely studied since. U.S. Pat. No. 5,853,918, to Tanno, et al., describes a carbon material composed of a layered structure and a turbostatic structure obtained by graphitizing a coke carbon material. U.S. Pat. No. 5,843,393 to Denton, et al., teaches using an amorphous carbon material as an electrode material in a rechargeable electrochemical cell. U.S. Pat. No. 5,451,477 to Omaru, et al., describes a non-aqueous liquid electrolyte secondary cell which includes an carbon anode comprising graphitic and non-graphitic carbons. Finally, U.S. Pat. No. 5,344,726 to Tanaka, et al., describes a carbon material comprising a crystalline core covered by an amorphous carbon layer. The carbon composite is formed by pyrolytically depositing carbon from an carbon containing carrier gas onto a carbon powder held in a closed vessel.
The use of ordered (crystalline and polycrystalline) carbons and true graphites has dominated production of lithium primary and secondary cells in recent years. However, multi-phase and disordered carbons are currently being investigated due to their observed superior reversible capacities as an active anode material comprising an inter-layer compound of a carbon material and lithium. In these inter-layer compounds, lithium enters into a space between the layers of carbon material during the charging process. Ordered-carbons are known to necessarily impose their structural order onto intercalated lithium due to the migration of the lithium into and between the carbon inter-layers as defined along the c-axis. This limits the overall electrochemical capacity of ordered carbon systems to 372 mAhr/g at standard temperatures and pressures (i.e. LiC.sub.6 formation). (The phenomenology and energetics of this interaction are reasonably well understood.)
Disordered (or crystallographically amorphous) carbons, on the other hand, lack the obvious "templating" which occurs in graphite and ordered carbons and have been shown to achieve reversible intercalation capacities in excess of 650 mAhr/g (e.g., on the 20th cycle). These disordered carbons seems to be behaving more like a sponge, picking up lithium irrespective of the influences, or lack of, local microstructural order.
This lack of crystallinity, i.e. disorder, however, can profoundly affect the rate of lithium loss at surface defects or functionalities. In these carbons, losses can vary from between 35% to 100% of the first-cycle reversible capacity due to irreversible electrolyte reduction at the surface of the carbon, as the result of parasitic surface reactions at the above mentioned surface defects, forming a solid-electrolyte interface (SEI), usually in the form alkyl lithium compounds and lithium salts. Similarly, particle microstructure can and does influence both intercalation and deintercalation rate capability of these carbons.
In order to better understand lithium insertion, transport, storage and trapping within the lithium secondary cell, disordered, "hard," carbons derived from polymerized methacrylonitrile ("PMAN")-divinylbenzene, have been uniquely processed to modify the surface microstructure of a large fraction of the disordered carbon powder. In particular, a technique herein identified as "laser-induced surface reconstruction" has been used to transform the surface of a PMAN disordered carbon into a more ordered, turbostatic state (herein defined as a carbon with a high degree of local correlation between adjacent ordered planes, i.e., with respect to Lc stack height, but whose orientational correlation with other similar grouping is low for length scales on the order of 2-3 times Lc). The modification is done because, as stated above, irreversible lithium losses associated with solvent decomposition and SEI formation are generally much lower for ordered carbons, whether turbostatic or fully graphitized.